Several modern high-speed serial data busses, normally used for digital communication between physically separated electronic devices, implement the well-known, time-tested “differential pair” signal line configuration to transmit and receive data. The differential pair typically consists of two signal lines, a positive (“p”) signal line and a negative (“n”) signal line, which normally exhibit opposite voltage polarities during normal data transmission. For example, a data value of ‘1’ is usually indicated on a differential pair with a voltage V+ on the “p” line, and a lower voltage V− on the “n” line. Similarly, to indicate a data value of ‘0’, the “p” line would hold a voltage of V− while the “n” line would exhibit a voltage of V+. Therefore, except during times in which the data value of the differential pair is in transition, the magnitude of the differential voltage between the two signal wires generally remains at about V+ minus V−. Use of a differential pair has long been known to exhibit high common-mode rejection, which allows the data being transferred to be unaffected by noise that is induced onto both signal wires of the differential pair.
In addition to transferring data, the modern serial data busses that utilize differential pairs for data transfer, such as Serial AT Attachment (Serial ATA), also utilize those same signal wires to indicate changes in the overall state of the communication link, such as to invoke data bus power management. To indicate these state changes, the two signal wires of the differential pair normally are driven so that the resulting differential voltage is substantially zero for specific periods of time. Driving the differential pair in this manner is commonly known as “squelch,” or “out-of-band” signaling. As a result, electronic devices connected to such a bus are usually required to detect the squelch state of the differential pair, which generally only lasts for a few tens or hundreds of nanoseconds.
At least two different methods have been employed in the past to detect the squelch state. For example, comparators that are used typically to detect the normal differential signals over the differential pair are supplemented with a “loss-of-signal” (LOS) output, which indicates that the differential voltage between the two signal wires has fallen below some predetermined threshold. However, the primary purpose of the LOS output is to detect catastrophic failures of the communication links, such as the signal wires of the differential pair being physically disconnected. Thus, the circuit that drives the LOS output is normally designed to ignore temporary periods in which the differential voltage falls below the threshold. As a result, the LOS output often reacts too slowly to reliably detect the squelch state.
Another previous method, which detects squelch in a more indirect fashion, is shown in FIG. 1. In that case, a comparator 1 with hysteresis is utilized in conjunction with a delay line 2 and a logical exclusive-or (XOR) logic gate 3. The “p” and “n” signal lines are connected to the positive and negative inputs of the comparator 1. While the differential pair transition between +V and −V, the output of the comparator 1 also continues to transition between high and low voltage levels. At each of those transitions, the output of the XOR gate 3 exhibits a logic HIGH level for the amount of the delay introduced by the delay line 2, indirectly indicating normal operation of the differential pair. Conversely, if the pair enters the squelch state for at least the delay period of the delay line 2, the output XOR gate 3 will go LOW, indicating the squelch state. Unfortunately, since the circuit of FIG. 1 is actually detecting the presence or absence of signal transition activity, as opposed to the presence of the squelch state, extended periods of time during which the differential pair resides at their proper +V or −V levels without transition will be incorrectly interpreted as squelch. Additionally, the hysteresis of the comparator 1 is required to prevent false transitions of the comparator 1 when the differential pair is actually in the squelch state. Unfortunately, most high-speed comparators, which would be required in the case of a high-speed serial bus, are designed with minimal hysteresis to facilitate higher signal switching frequencies.
From the foregoing, a need presently exists for a reliable circuit that detects the squelch state of a differential signal pair. Such a circuit would directly detect the squelch state quickly, without relying on a minimum level of transition activity on the differential pair while not in the squelch state.